In a letter to Treasury Select Committee chairperson Andrew Tyrie, dated 20 May but only released today, FCA chief executive Martin Wheatley said that the FCA is working with the BoI to help protect consumers.

All mortgages affected by the rate changes were sold prior to regulation in 2004, which means they are out of the FCA's jurisdiction; however, Mr Wheatley said that the FCA is continuing to apply pressure on the bank.

In the letter, Mr Wheatley also revealed that affected borrowers who were sold a mortgage through an intermediary may be able to take legal action against that intermediary if "they believe they had received negligent advice".

"Customers concerned have the right to complain to the firm, to the intermediary that sold the product and, in most cases, to the Financial Ombudsman Service," he said.